There are these brass coloured bolts at the side of the window into which an Allen key fits. As far as I remember size 6 (6mm). These bolts have a marked/engraved slit like the hand on a watch. If this hand shows toward the frame (which holds it tight/catches it when the window is closed) then the tightness is not very strong. When you use the Allen key you can turn the bolts, the tightest position would be when the hand shows to the window. Don't overtighten it, work bit for bit until a sheet of paper or a playing card is not falling/slipping through the sealant strips.